Black Hills ride. I have done the WRATH ride the last two years and it is great. There were several riders from Colorado riding and they loved it because it was challenging and not crowded.
It is getting bigger each year but still around 100 riders. There are three choices of rides each day from about 40 miles to 100 the first day and 30 to 70 miles the second day. Vertical climbs of up to 7,000 feet on the 100 mile and 6,000 feet on the 75 mile ride.
There are rest stops every 10 to 12 miles and they really treat the riders well.
google MS WRATH ride, Weekend Ride Around The Hills.

I am currently in Austin TX visiting my daughter and grand daughter. There is an airport in Rapid City served by Northwest, United and Delta.
If you drive it is about 1230 miles if you don't take I 35, I always take Hwy 83. About 21 and 1/2 hours driving time. I have made that trip at least twice a year for the last 12 years.

I googled the ride and this year it goes through the southern hills, I don't think there is a more scenic ride in the country with the exception of the Rockies. Lots of climbs and descents, I prefer to climb, the descents get a little hairy at times.